People who filed just before or during the outage don't need to do anything, the IRS said. E-file providers, such as H&R Block and TurboTax, held the returns they received during the outage, and have now started sending those tax returns into the IRS, the agency noted.Statistics on how many people have filed already aren't available. But at this time last year, roughly 27 million filers' returns had been processed -- and almost of all them had been e-filed. And more than 19.5 million of those filers were due refunds, averaging $3,366.Taxes are due this year on April 18.

Outdated technologyThe IRS did not elaborate on what caused the outage, saying that is was "continuing to examine the underlying cause ... as well as monitoring any follow-up issues." It still believes a "hardware failure" occurred.

The agency's systems aren't exactly at the cutting edge of technology. Thanks to persistent budget cuts, the agency's efforts to modernize its technologies have been held up, IRS Commissioner John Koskinen told a Senate panel in February 2015.Related: IRS customer service could soon get a lot worse​"We're running applications we were running when John F. Kennedy was president," Koskinen told lawmakers.He also said some IRS systems still use the COBOL programming language, which Computer Worldonce described as "a programming dinosaur that was last hot in the 1980s."When one senator asked how the agency could still be using such old systems when it spends over $2 billion a year on technology operations, Koskinen explained that the money has been going into upgrading its systems, which were customized for the IRS in the 1950s and 1960s.